The objectives of this project are to define the epidemiology, risk factors, transmission kinetics and pathogenesis of Chlamydia trachomatis infections in different population settings and different disease states using new molecular amplification assays. To address this objective, we implemented non-invasive screening with molecular assays for C. trachomatis and Neisseria gonorrhoeae, and documented high rates of infection in sexually active adolescents in diverse cultural settings in six countries. Prevalence rates of chlamydia ranged from 3.8% in rural Zimbabwe and Uganda; 9.2% among students in St. Petersburg, Russia; 7.7% in individuals recruited from barrios in Lima, Peru; and 9.5% in patients in Fuzhou, China. Women generally had higher prevalence rates of chlamydia in 16 to 20 year olds compared to men with prevalence declining in both genders in the older age groups except for China, where the prevalence remained elevated in all age groups up to 40 years old. The documented high rates of chlamydia in these countries raises serious concerns about the resurgence of STDs that may reflect a rise in high-risk behavior which may subsequently lead to further HIV transmission. In the US, among 23,010 non-health-care-seeking female Army recruits at Fort Jackson, SC, prevalence for chlamydia was 9.51% for four years. There was a progressive increase from 8.51% to 9.92% over the four-year screening period (p = 0.018). Significant risk factors for infection included African American ethnicity, young age (< 25 years), southern U.S. residence, more than one sex partner, a new sex partner and history of any sexually transmitted disease. Consistent condom use was found to be protective. The study was extended to examine the frequency of chlamydia among male military recruits. The prevalence of chlamydia and gonorrhea was 5.3% and 0.6%, respectively, in 2,245 male recruits. African American race, new sex partner, history of trichomonas and the presence of urethral symptoms were strongly associated with chlamydia infection. In summary, the prevalence of chlamydia was found to be high in both men and women entering the US military. Screening on the basis of symptoms alone would miss the majority of chlamydial infections in both men and women, warranting more routine screening upon entry into the Army. To determine the magnitude of infection in inner city populations a household survey in Baltimore was conducted of 579 adults aged 18 to 35. 7.9% of the population had either untreated gonococcal or chlamydial infection with an estimated prevalence substantially higher among African American females (15%). In a related study of 434 patients aged 18 to 31 years attending the Johns Hopkins Emergency Department, 13.6% of patients were infected with chlamydia or gonococcal infection. Only a quarter were initially suspected and treated for their STD in the Emergency Department. Since previous studies had supported an association between atherosclerosis and infection with CMV or C. pneumoniae, we undertook a study to determine whether both infections together would contribute to atherosclerosis in a dose-dependent manner. ApoE-knockout mice were infected with murine CMV and C. pneumoniae to determine whether infection with multiple pathogens increases lesion size to a greater extent than either pathogen alone. At 16 weeks of age, infection with CMV alone, C. pneumoniae alone, or both CMV and C. pneumoniae increased lesion size 84% (p < 0.001), 70% (p < .0001), and 45% (p < .01), respectively compared to controls. The CMV-induced increase in circulating levels of interferon-gamma may have contributed to this increase in lesion size. However, the increase in interferon-gamma due to CMV may have subsequently reduced the infectivity of C. pneumoniae in the dually infected animals since interferon-gamma is inhibitory to the growht of chlamydia. Thus, the animals not previously infected with CMV might develop a more aggressive C. pneumoniae infection than the animals that were previously infected with CMV, resulting in the observed atherosclerotic effects. In summary, this investigation provides further evidence that infection with either pathogen can causally contribute to the development of atherosclerosis.